The invention relates to a process for controlling a programmable or program-controllable hearing aid for in-situ adjustment of said hearing aid to the optimum gain in one or more frequency bands, with due consideration of any possible acoustical feedback, as per the preamble of claim 1.
It is well known that with hearing instruments, be it with BTE hearing aids that are connected to the ear canal by means of a small-diameter plastic tubing and an earmold, or with an ITE hearing aid inserted deeply into the ear canal with its earmold or otoplastic, acoustic feedback is possible from the residual cavity between the earmold and the timpanic membrane to the microphone, either by a less than perfect fit of the earmold in the ear canal or by a small venting tubing provided for pressure relief, or both.
This has for example been described in "HEARING INSTRUMENTS, Vol. 42, Nr. 9 1991, pages 24, 26".
Additionally, U.S. Pat. No. 5,259,033 and its European counterpart EP 0 415 677 A2 disclose a hearing aid with an electric or electronic compensation for acoustic feedback. Particularly, the hearing aid includes a controllable filter in an electrical feedback path, the characteristics of which are calculated and controlled to model the acoustic coupling between the earphone and the microphone of the hearing aid using a correlation method.
A noise signal is injected into the electrical circuit of the hearing aid and is used for adapting the filter characteristics to accommodate changes in the acoustic coupling.
The coefficients for controlling the filter characteristics are derived by a correlation circuit.
Furthermore the WO 93/20668, published with abstract and claims in english and drawings only discloses in principle the same circuitry, further including a digital circuit which carries out a statistical evaluation of the filter coefficients in a correlation circuit and changes the feedback function adaptively. The compensation covers the entire audible frequency range.
WO-A-9005437 and U.S. Pat. No. 4,185,168 are both further examples of automatic systems for reducing feedback problems, when they occur during normal operation. For this purpose simply additional complex circuitry is used in the hearing aid and additional filters are required, which will effect the entire frequency band where and when they are activated.
Many of the more modern hearing aids are capable of varying the gain in order to adjust to the actual sound environment and the actual hearing loss. This can be done in one or more frequency bands.
Most hearing losses are characterized by "recruitment". In other words, weak sounds cannot be detected and powerful sounds are heard as normal hearing people would hear them. Traditionally, these hearing losses are fitted with hearing aids having a fixed gain. This gain is typically too low at weak sound levels and too high at powerful sound levels.
To compensate more ideally for this kind of hearing loss the hearing aid should have high gain at weak sounds and zero or low gain at powerful sounds. Such types of hearing aids typically have high gain in quiet environments which increases the risk of acoustic feedback. The gain at which feedback occurs depends primarily on the quality and shape of the earmold.
However, until now the most common way to solve the problem of an unsatisfactory earmold that caused unacceptable acoustic feedback was to throw it away and have a new one made. This means that no one ever knew what was wrong with it and exactly how bad the earmold was.
Obviously poor earmolds cause considerable problems in case of severe hearing losses and the then necessary high gains. In order to avoid feedback with an earmold that cannot be made better the hard-of-hearing has the only choice to turn down the volume control for the entire frequency range.
Generally, there are more and more programmable, program-controllable or programmed hearing aids most of which could be reprogrammed for one or more frequency bands or channels by an external programming unit for one or more transmission characteristics and, mostly, adapted at the same time to the actual hearing loss of the wearer.
Unfortunately, when in-situ programming and fitting of a hearing aid of this type, there are presently no instruments to detect any acoustic feedback combined with an automatic testing process to adjust the hearing aid to an insertion gain that avoids acoustical feedback and/or indicates whether for the amplification/gain required for a specific hearing loss the earmold is fitting well enough in the ear canal. This would have the result that at this maximum gain for the specific hearing threshold level no acoustic feedback would occur, indicating whether this earmold has the required quality of fitting inside the ear canal for the specific gain required.